Objective: To investigate effects of Relief ACP on dentin microhardness and surface morphology of extracted human teeth compared to that of Satin Finish. Methods: Twenty dentin specimens were prepared from extracted human molars by grinding from enamel surface until the dentin was exposed and then polished with Gamma Alumina; the smear layer was removed by treating the dentin surface with 15% EDTA solution for one-minute. The sample surface was measured for Knoop Hardness Number (KHN) with a Leco Microhardness Tester (M-400-H1, St, Joseph, MI). The specimens were randomly assigned to three groups. Group A (N=4) served as the Control (100% humidity). Group B (N=8) was treated with Relief ACP (Discus Dental, Culver City, CA), while Group C (N=8) received treatments with Satin Finish (Discus Dental). The samples received 28 treatments (30-min/ea); prior to each treatment the samples were immersed in pooled human saliva for 20 minutes. The KHN was measured after the last treatment, and the specimens were then processes for the SEM evaluation. The KHN data were analyzed using the One-way ANOVA and Student-Newman-Keuls methods. Results: There were no significant differences in the KHN values among the three groups before and after treatments. The changes in KHN were statistically different (p=0.032); however, there were no significant within-treatment differences for any of the three groups. The SEM evaluation showed deposits inside of the exposed dentin tubule openings in the specimens treated with Relief ACP, and most of the openings appeared fully blocked by the deposits. Such deposits were not observed in the Control samples and they were less evident in the Satin Finish group. Conclusion: The treatment with Relief ACP or Satin Finish does not change surface microhardness of human dentin, and treatments with Relief ACP produce deposits inside of the exposed dentin tubule openings.